As of today, downy mildew has not been confirmed in Ohio or in the Great Lakes region. However, since we usually see the first symptoms of the disease on cucumber around the 4th of July, growers in high risk areas should start applying fungicides effective against downy mildew (see table below) on 5-7 day intervals. Use the shorter intervals in cooler, high moisture conditions. Stretch the interval if the weather becomes hot and dry, especially if the disease is not reported nearby. Always tank mix with chlorothalanil or mancozeb. Alternate products with different modes of action (FRAC codes, see table below).
Early season downy mildew risk for Ohio cucumbers and melons is highest in counties in approximately the northern third of the state.
Downy mildew on cucumber
Downy mildew on cantaloupe
Click to enlarge
Fungicide recommendations great even for Guatemala conditions.